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FAQ
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XSOC FAQ (Frequently Asked Questions) 1.00 3/1/2000
Edited by Jan Gray (jan@fpgacpu.org)
Copyright (C) 2000, Gray Research LLC. All rights reserved.
The contents of this file are subject to the XSOC License Agreement;
you may not use this file except in compliance with this Agreement.
See the LICENSE file.
FPGA DEV TOOLS
Q. Has anyone built XSOC under Foundation 2.1i?
A. Yes, but see issue #20.
Q. I use Viewlogic tools. Can I edit and rebuild XSOC?
A. We don't think so. If you do manage to do so, please let
us know how it goes.
Q. Schematics are a drag. Do you have a version in VHDL or Verilog?
A. Yes, as of release beta 0.93, XSOC includes a Verilog version of
the project. It has been built under FPGA Express and it runs
in an XS40 board. The whole design, including xr16, is about
1,000 lines of Verilog. It has an almost identical footprint,
and so the entire XSOC system still fits nicely in an XC4005XL.
WHICH XESS XS40 BOARD
Q. I am going to buy an XESS XS40 board. Which one do you recommend?
An XS40-005XL or an XS40-010XL? A standard 32 KB RAM version, or
the '+' board with 128 KB RAM?
A. Our advice is to buy the XS40+ -010XL board, with the larger XC4010XL
FPGA and 128 KB, if that fits into your budget. The XSOC design
fills an XC4005XL to the brim, so you might find yourself getting
tight for space if you buy the -005XL. And by also springing for the
extra RAM, you'll be able to tackle larger projects and programs,
more peripherals, higher resolution or color video, and so forth.
>16-BIT ADDRESSING
Q. A 16-bit address space is a drag. How about an MMU?
A. It would be very easy to add an MMU to, for instance, map
A[15:12] to a physical address PA[25:12], plus a 'writeable
bit'. See section 6.4 of the xr16 Specifications for a sketch
of one possible specification. Actual implementation is left as
an exercise.
Q. A 16-bit address space is a drag. How about a 32-bit version?
A. We have a 32-bit Verilog version "in the labs", xr32, but it has not
yet been simulated, nor run in hardware (we've been busy shipping
the present kit). The xr32 has the same 16-bit instruction format
as xr16, although two of the three reserved opcodes are allocated to
ll and sl (load/store longword) instructions. Stay tuned.
PORTING TO OTHER FPGA ARCHITECTURES
Q. Is XSOC limited to XC4000E-derived FPGA families?
A. The current schematic version is tuned and floorplanned for
these FPGA architectures. It makes use of device features including
H-function generators and it assumes an abundance of BUFTs (three-
state buffers driving long-line buses). The Verilog version makes
fewer such assumptions and will likely prove easy to retarget,
at least to Virtex-derived FPGA families. We hope that someone
will attempt to build XSOC, xr16, and/or xr32 on Altera families.
LICENSING
Q. My organization would like to use xr16 in an application that is not
"non-commercial educational and research purposes". What to do?
A. Such use is not permitted by the XSOC License Agreement (see the
LICENSE file). As of this writing we are evaluating whether to
license XSOC for other purposes. Please contact us directly (email
xsoc@fpgacpu.org) to discuss your specific situation.
SUPPORT
Q. What kind of support can I expect?
A. If you assume none whatsoever then you won't be disappointed.
A. Reread the XSOC License Agreement.
A. The user community (fpga-cpu@egroups.com) may be able to help you.
See the Help! section in the Getting Started Guide.